1. Field of the Invention
The present invention relates to a method for distillation of sulfur for the preparing radioactive phosphorous nuclide. More particularly, the present invention relates to an economically favorable and efficient method in which sulfur is converted into radioactive phosphorous nuclide by neutron irradiation while unreacted sulfur is separated from the radioactive phosphorous nuclide by distillation and recovered at high efficiency, with the radioactive phosphorous nuclide remaining high in purity.
2. Background of the Related Art
Emitting β− radiation, nuclides such as 32P and 33P find many applications in various fields, including medical treatment, synthesis of labeling compounds, bioengineering experiments, etc.
The phosphorous nuclide (32P) can be prepared by the nuclear reaction of 32S(n,p)32P or 31P(m,γ)32P In spite of its guaranteeing very simple chemical treatment after neutron irradiation, the (n,γ) reaction is only adopted in special cases because the uses of the resulting 32P are limited due to its low specific radioactivity. For use in medical treatment or research experiments, the phosphorous nuclide 32P is usually obtained by separating it from the sulfur target after 32S(n,p)32P nuclear reaction.
Depending on physical and chemical statuses of the sulfur, separation of the 32P generally resorts to the following methods.
32P may be purified by a wet extraction method in which strong and weak acids are used to extract the phosphorus nuclide from the sulfur target. According to the wet extraction method, 32P is extracted from finely powdered sulfur irradiated with neutrons in boiling water in the presence of acid [Samsahl, K., Atompraxis 4, 14, 1958; Razbash, A. A. et al., Atomnaya Ehnergiya 70(4), 260, 1991]. In this regard, 2-octanol is used as a wetting agent. This method suffers from the following disadvantages. The extraction yield varies with the particle size of the irradiation target sulfur and is significantly decreased when the target is melted or solidified due to the exothermal heat during neutron irradiation. Additionally, the use of acid induces impurities and leaves much solid waste behind, thus completion of the extraction requires additional purification processes.
Alternatively, 32P may be prepared by irradiating the sulfate or polysulphide target with neutrons, dissolving the target in water, and then adsorbing or coprecipitating the 32P thus formed. Because it requires multi-stage processes and produces low recovery yields, this method is scarcely used.
Suggested as an alternative which can solve the above problems were sulfur distillation methods which are generally classified into: atmospheric distillation in which sulfur is distilled at as high as 500° C. in a nitrogen atmosphere; vacuum distillation in which sulfur is distilled at as low as 180-200° C. under a pressure of 1-10 mmHg [Gharemano, A. R. et al., Radiochemical and Radioanalytical Letters Hungary 58(1), 49, 1983, Ye. A. Karelin et al., Applied Radiation Isotopes 53, 825-827, 2000]. The former employs an inert gas as a carrier in order to reduce the possibility of fire. In the latter method, distillation is carried out at a temperature lower than the ignition point of sulfur by reducing the pressure. These distillation methods are advantageous in that products of high purity can be obtained since no reagents are added during the separation of phosphorous nuclide from sulfur. However, the methods require facilities such as a vacuum system, a gas-feeding apparatus and a cooling apparatus in order to distill the sulfur irradiated with neutrons in hot cells or glove boxes, as well as require the pressure and temperature to be controlled in relatively narrow ranges. Additionally, where concentrated sulfur is used, it is difficult to recover the whole amount of very expensive sulfur, which brings about an economic loss.
Therefore, there remains a need for an improved method that can prepare phosphorus nuclides of high purity easily and very economically.